How do you ask guest to bring gifts to a 50th birthday party?

I am not sure they is any proper way to ask for a gift.A gift ,by its very nature is voluntary.
I believe most people would be inclined to bring a gift to a birthday party.
If you feel compelled to say anything regarding a gift,I would say please no gifts over $50.00.
However,I stand by my original statement that it would not be proper etiquette to ask for gifts.